1. Field of the Invention
The present invention relates to methods for producing microelectronic devices that are protected from physical damage induced by external conditions. In particular, the invention relates, but not exclusively, to semiconductor devices having (i) a tamper resistant coating (TRC) to protect the device from physical or electromagnetic inspection and (ii) an encapsulation package to protect the device from physical damage induced by extreme operating conditions and/or the surrounding environment.
2. Related Art
Tamper resistant coatings (TRCs) are well known in the art for providing a physical barrier to prevent inspection of, and tampering with, the underlying circuitry and contents of electronic components.
For example, protective coating processes using application of liquids, are described in U.S. Pat. No. 5,399,441 to Bearinger et al. and U.S. Pat. No. 5,258,334 to Lantz. Such liquid application processes, however, tended to be disadvantageous in that they typically involved (a) processing temperatures that could be detrimental to delicate circuitry, and (b) applying coatings before circuit connections are made, thus tending to make the resulting device less tamper resistant.
Processes and systems for coating electronic circuits with protective coatings and security coatings using a thermal spray are also generally known. Examples of such processes are described in U.S. Pat. Nos. 5,877,093; 6,110,537; 5,762,711; and 6,319,740 all to Heffner et al. and fully incorporated herein by reference. As compared to processes involving liquid application, such thermal spray processes typically use temperatures less likely to be detrimental to delicate circuitry and provide a better coverage of coating at lower cost. Also, the thermal spray coatings are typically applied after circuit connections are made, thus improving the tamper resistant properties of the resulting device.
However, applying protective coatings or tamper resistant coatings using a spray, sputter, deposition or other floating particle application process are susceptible to, for example: (i) leaving uncoated areas underneath certain surfaces of the device; and/or (ii) leaving pockets of air in or under the coatings of the device when the device is subsequently encapsulated or coated with an encapsulation material. Uncoated areas and/or air pockets may occur at the same locations on devices coated with a sprayed particle process, most notably, under or around wire bonds establishing electrical connection to semiconductor device. An example of uncoated areas that may result from coatings applied using a spray process is discussed in greater detail below in reference to FIGS. 2A and 2B.
The patents to Heffner et al. disclose coatings applied by thermal spray process to circuits in a ceramic package. Air gaps and exposed (uncoated) areas in devices with ceramic packages are typically not as problematic since ceramic packages are typically sealed on all sides and since little force or pressure is ever applied on circuit areas such as the bond wires. However, devices enclosed in ceramic packages may not be suitable for high acceleration and/or shock applications since the ceramic packages have a tendency to fracture or break under stress or impact. Consequently, it is preferable to use a laminate substrate for a shock resistant microelectronic circuit. In addition, laminate substrates are less expensive and are easier to fabricate than their ceramic counterparts and thus are better suited for high acceleration and/or shock applications.
However, the uncoated areas and/or air pockets that may result from application of protective and security coatings may be more problematic in a laminate substrate device than in ceramic packages. Where some of the device surfaces remain uncoated, the device may be susceptible to corrosion resulting from moisture, ionic content and voltage present at the exposed areas. In addition, gaps present under surfaces of the coated device (e.g., under wire bonds) may pose additional problems in that; physical stresses and forces encountered by the coated device can sometimes cause electrical connections to severe or come loose near gap.
In an attempt to reduce exposed areas after a thermal spray process, an encapsulation layer may be applied to the device. However, during this process, air pockets may be left in the circuit package. Air pockets can be detrimental because leakage of some air during a cure process of the encapsulation layer tends to form void defects in the package and lead to device failures from, for example, humidity exposure, etc. Additionally, entrapped air may increase internal stress during temperature cycles and thus lead to higher failure rates.
Consequently, there is a need for a low cost, reliable, microelectronic device having improved tamper-resistant characteristics and a high tolerance to shock and vibration, and method for making the same.